According to the dry electrophotographic process in general, the printing or copying is performed in such a manner that an image is electrophotographically formed on a photosensitive drum, the electrophotographic image is transferred to a transfer paper and a thermoplastic resin powder (hereinafter referred to as a toner) containing a pigment having a charge opposite to the charge constituting the electrophotographic image is attached to the electrophotographic image part on the transfer paper to fix the image as a visible image.
Conventionally, inorganic photoconductive materials such as selenium, zinc oxide and cadmium sulfide are used as the electrophotographic photoreceptor, however, these are not necessarily satisfied with respect to the properties such as thermal stability, durability, processability and flexibility. In recent years, accordingly, organic photoconductive materials are prevailing. In particular, a function separated photoreceptor comprising a charge generating layer containing an organic photoconductive material in a transparent organic resin cured product and a charge transporting layer containing a charge transporting material in a transparent organic resin cured product, which are laminated in this order, is mainly used. The shape of the photoreceptor includes sheet, belt and drum, however, a predominant photosensitive drum comprises a cylindrical drum core made of a conductive material such as a metal, having laminated on the circumferential surface thereof a charge generating layer and a charge transporting layer in this order.
The charge generating layer and the charge transporting layer both are required to have a high performance with respect to the transparency, the electric charge holding effect and electrostatic charge acceptability in a dark place, and the like. In particular, the charge transporting layer is an outermost surface of the photoreceptor and therefore, is further required to have excellent properties such as high abrasion resistance, high stain resistance (toner releasability) and corona resistance.
In order to remove toner not transferred to the transfer paper but remaining on the surface of the photoreceptor, a cleaning blade or brush is usually put into contact with the surface of the photoreceptor. If the surface of the charge transporting layer is inferior in the abrasion resistance, due to sliding in contact with the cleaning blade or brush, the surface of the charge transporting layer is readily roughened or wears to cause deterioration of the properties.
In the case where a corona discharge device is used for charging the photoreceptor, ozone is generated due to the corona discharge. The ozone destroys the bonding of the organic resin in the charge transporting layer of an organic transparent cured product and accordingly, the surface of the charge transporting layer is readily roughened to cause deterioration of the properties.
If the surface of the charge transporting layer is roughened due to abrasion and/or corona discharge, the effect of cleaning the residual toner is naturally reduced and the photosensitive properties are deteriorated, which adversely affects the image. Thus, if the surface of the charge transporting layer is liable to be roughened, it gives rise to the reduction in the number of sheets printable by one photoreceptor, namely, the press life.
In recent years, taking account of transparency and abrasion resistance, a special acrylic resin or special polycarbonate resin is being used as the transparent organic resin cured product of a charge transporting layer in many cases, however, these are still not yet satisfied with respect to the capabilities such as abrasion resistance, stain resistance or corona resistance.
As the method for reducing the abrasion loss on the surface of the photoreceptor, a large number of proposals have been made for peripherals of the photoreceptor, such as cleaning blade or toner. JP-A-57-128376 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") proposes a method for reducing the abrasion loss of the photosensitive drum surface by fixing a silicone oil to the surface of the cleaning blade to thereby reduce the coefficient of friction between the photosensitive drum surface and the cleaning blade, however, the silicone oil fixed is not retained and the durability cannot be improved as expected. JP-A-4-16855 proposes a method for reducing the abrasion loss of the photosensitive drum by mixing a thermoplastic fluororesin into the toner to thereby reduce the frictional resistance between the photosensitive drum surface and the cleaning blade, however, the cost of toner is increased and the range of the fixing conditions is disadvantageously narrowed.
For improving the durability of the photoreceptor surface from the material side, a method of applying a silicone resin coating as the surface protective layer has been proposed. JP-A-2-148043 proposes a method for improving the durability of the photoreceptor by forming a cured product of a silicone resin coating containing an organic acid and/or an inorganic acid, as the protective layer on the photosensitive layer. The silicone resin has properties such that as compared with the organic resin, the cured coating thereof has high hardness, is improved in the abrasion resistance and is difficult of erosion by ozone generated on the corona discharge. This silicone resin is a hydrolysate of alkoxysilane and necessary to be heated for the curing even if a curing catalyst is used in combination. In the Example of this patent publication, 1 hour was spent for the curing at 100.degree. C. and thus, the silicone resin is deficient in that the processability is inferior. JP-A-6-11853 proposes a method of using a cured product of a coating material mainly comprising silica for the charge generating layer to impart abrasion resistance to the surface of the layer and providing the charge generating layer on the outermost circumference of the photosensitive drum. When the outermost circumference is a charge generating layer, advantageous effects result such that the photoreceptor surface is electrified to the positive charge and the problem of the generation of ozone can be evaded. However, for forming a cured product having sufficiently high abrasion resistance from the coating material mainly comprising silica, heat treatment at 200.degree. C. or higher is usually necessary and this may impair the properties of the photoconductive material. In the Example of this patent publication, a cured product was formed by the heat treatment at 80.degree. C., however, the reason why the low-temperature curing was achieved is not described. Further, the cured product obtained does not have toughness and is disadvantageous in that cracks are readily generated. JP-A-2-148043 and JP-A-6-11853 both have an object of improving the abrasion resistance and the corona resistance by forming a silicone resin or silica harder than the organic material on the outermost circumference of the photoreceptor, however, the stain resistance against residual toner (releasability) is not taken into consideration in these patent publications.